1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on an image pickup device such as a CCD (Charge Coupled Device) sensor or CMOS (Complementary Metal Oxide Semiconductor) sensor and more particularly to compact imaging lenses which are mounted in PDA (personal digital assistants) such as mobile phones and the others.
2. Description of the Related Art
Today most mobile phones have a camera function and recently mobile phone models with a high resolution camera function comparable to a digital still camera have been introduced into the market. The pixel size in an image pickup device is very small and the pixel pitch is less than 1.4 microns. On the other hand, as mobile phones become smaller and thinner, there is a demand for smaller imaging lenses and in order to cope with the demand for compactness of imaging lenses and higher resolution of image pickup devices, the need to improve the aberration correction capability of an imaging lens is becoming stronger.
Conventionally, imaging lenses for mobile devices such as mobile phones and smart phones have been made of aspheric plastics, in which aberration correction is made mainly by a lens array combination, namely combination of lens power and lens shapes. Chromatic aberration has also been corrected similarly and examples of such imaging lens configurations are described in JP-A No. 2007-264180 (Patent Document 1) and JP-A No. 2010-197665 (Patent Document 2).
On the other hand, for chromatic aberration correction, a method which uses a diffractive surface is known and it has been already applied to zoom lenses (JP-A No. H10-213744 (Patent Document 3) and JP-A No. H11-23968 (Patent Document 4)). However, it has been rarely applied to light and compact fixed-focus cameras, namely imaging lenses for mobile phones and smart phones.
Considering that image pickup devices tend to provide higher resolution and the temperature dependence of plastic lens refractive index is high, a high accuracy chromatic aberration correction method should be adopted for imaging lenses for mobile phones as well.
The diffraction method takes advantage of the fact that the Abbe number (d-ray) of a diffractive surface is −3.452 (negative value), in which a single lens is used to implement an achromatic mechanism which would be implemented using two (positive and negative) lenses in a conventional technique. It is also a useful technique in an effort to decrease the number of lenses and shorten the total optical length of the imaging lens.
Next, related art techniques will be described.
The imaging lens described in Patent Document 1 is comprised of five lenses which are located in the following order from the object side: a first lens a with positive refractive power, a second lens as a negative meniscus lens with a concave surface on the image side, a third lens as a positive meniscus lens with a convex surface on the image side, a fourth lens with negative refractive power, and a fifth lens with negative refractive power having a concave surface on the image side near the optical axis. This technique is designed to ensure chromatic aberration correction and telecentricity, in which a low-dispersion material is used for the first lens, a high-dispersion material is used for the second and fourth lenses and for the fourth and fifth lenses, a conditional expression concerning lens thickness and inter-lens distance and a conditional expression concerning focal length are derived to correct chromatic aberration. The F-value is 2.8 or so and the half-angle of view is 31.9°, though both the F-value and half-angle of view are insufficient for adaptation to a high density image pickup device. In addition, since a glass material is used, the technique is disadvantageous from the viewpoint of cost reduction.
Similarly the imaging lens described in Patent Document 2 includes five lenses arranged in the following order: positive, negative, positive, positive and negative power lenses. In this technique, the second lens is mainly used for chromatic aberration correction, namely the second lens is largely responsible for chromatic aberration correction, so it should have large refractive power and manufacturing tolerance is tight. The F-value is 2.8 or so, which is insufficient.
The above two techniques only use an array of lenses to make chromatic aberration correction, but correction of chromatic aberration and other various types of aberrations by a lens array combination has limitations and design freedom in such techniques is very low.
On the other hand, Patent Document 3 describes a two-group zoom lens in which each group has a diffractive surface to correct axial chromatic aberration from a telescopic end to a wide-angle end. As far as comparison is made on the telescopic end and wide-angle end, it is hard to say that the use of two diffractive surfaces is more advantageous in spherical aberration correction than the use of a single diffractive surface. Patent Document 4 describes a three-group zoom lens in which one group has a diffractive surface to correct axial chromatic aberration and chromatic aberration of magnification from a telescopic end to a wide-angle end. Whereas, as mentioned above, zoom lenses use a diffractive surface to ensure that chromatic aberrations at the telescopic end and wide-angle end are corrected, fixed focal length lenses rarely use a diffractive surface.